Kelvin, Perry and the Age of the Earth

The 19th-century scientific community grappled at length with the question of the age of the Earth, a subject for which a definitive answer did not arrive until the refinement of radiometric dating in the mid-20th century. The most famous—and famously wrong—estimation of the Victorian era came from the renowned physicist William Thomson (1824-1907), known from 1892 as Lord Kelvin.

The story of Kelvin and the age of the Earth is often told as a David-and-Goliath struggle, with geologists playing the role of underdog, armed only with the slender sword of geological reasoning, while Lord Kelvin bludgeoned them with the full force and prestige of mathematical physics. Kelvin's eventual comeuppance is often taken as evidence that simple physics ought not to be applied to complex geological problems. But there are many simple physical models that have had great explanatory power in geology.

Many people believe that Kelvin's calculation failed through his ignorance of radioactivity. Here, we examine Kelvin's approach and show that this was not where his error lay. The flaw in Kelvin's thinking was divined by one of his own assistants, a scholar, educator and inventor named John Perry, who attempted and failed to convince the establishment of the day that enhanced heat transfer in the Earth's interior—by convection or some other means—could reconcile the geological and the physical arguments. Today it is possible to see how Perry's ideas could have advanced the study of the Earth considerably, had geologists understood and appreciated them.